1. Field of the Invention
This invention relates to the field of polymeric coatings and particularly to the field of flame-resistant polyimide coatings.
2. Description of the Prior Art
Many structures such as fiberboard or cardboard containers have poor flame resistance. Although their flame resistance can frequently be improved by incorporating additives in the fiberboard formulation, these additives generate toxic volatiles when pyrolyzed. Insulative foam can be attached to the exterior of such structures, but the large volume of the foam reduces the useful space.
Another approach used to improve the flame resistance of structures is to paint them with intumescent coatings. These are paints which contain an intumescing agent. When heated, the paint expands and insulates the structure from the flame by increasing the distance between the flame and the surface of the structure. One intumescent paint which has been reported (NASA Tech Briefs, Summer 1979, page 224) has a fluorocarbon latex resin base plus a pigment and miscellaneous additives. This waterbased paint has approximately 40% of an intumescing agent such as sulfanilamide, melamine pyrophosphate, or polysulfonamide derivatives of polycyclic aromatic compounds.
Prior art intumescent coatings suffer from one or more of several shortcomings. In some cases, a toxic gas is generated during intumescing which increases the hazard to persons present during a fire. Frequently the efficiency of the coating decreases in the presence of moisture because of the ionic character of the intumescent agents used. Additionally, the expanded coating may be weak and friable, causing it to erode rapidly from the structure's surface by the impinging flame and hot gases.
With respect to production of plastic foams, U.S. Pat. No. 3,300,420 to H. E. Frey describes a polyimide foam and a method of producing the foam from aromatic anhydrides and isocyanates. According to the Frey patent, the reactants are mixed together and then heated to 300.degree. F. to about 700.degree. F. to form either a solid foam product or a prepolymerized intermediate useful for forming solid polymeric products.
More recently, U.S. Pat. No. 4,184,021 to Sawko, Riccitiello, and Hamermesh (one of the present inventors) describes a method of preparing a polyimide foam in which the required heating is obtained by including in the reactants furfuryl alcohol and phosphoric acid. During mixing of the reactants, the furfuryl alcohol and phosphoric acid produce a vigorous exothermic reaction which provides the heat that is necessary for the formation of the polyimide structure.
U.S. Pat. No. 4,303,727 to Hamermesh, Tung and Hogenson discloses an intumescent flame-resistant polyimide coating produced from a product resulting from mixing an aromatic polyisocyanate, an aromatic polycarboxylic compound and 25 to 60% furfuryl alcohol at a temperature less than 80.degree. C. When a structure coated with such product is exposed to flame, the coating intumesces to form a flame-resistant foam which protects the structure.
The resulting intumescent coating protects a flammable substrate by expanding and providing a fire-resistant insulative barrier between the fire and the substrate.
However, for an intumescent coating to be useful, it must not foam prematurely when exposed to heat sources (not flame) at relatively low temperatures. Thus, for many applications, it is desirable that the coating should not foam or intumesce when exposed to ambient conditions in a temperature range of about 50.degree. C. to about 110.degree. C.
The coating of above U.S. Pat. No. 4,303,727, although effective and providing a fire-resistant foam, has the disadvantage that such coating has a relatively low kickoff temperature, or temperature of intumescence, of the order of about 80.degree. C. It was believed that such premature foam generation was the result of the formation of polyimide and the generation of carbon dioxide. It now is believed that the reaction which results in such relatively low kickoff temperature apparently is due to the polymerization of furfuryl alcohol in the composition of the patent, which occurs at about such temperature.